1. Field of the Invention
The present invention relates generally to an apparatus and method for generating a space-time trellis code (hereinafter referred to as “STTC”) in a mobile communication system, and in particular, to an apparatus and method for generating STTC for maximizing space-time diversity gain and coding gain.
2. Description of the Related Art
Unlike a wired communication system, a mobile communication system based on a wireless communication system uses limited frequency resources. The wireless mobile communication system must use a multilevel modulation scheme in order to transmit information at high speed. In the multilevel modulation scheme, a data rate can be increased as the number of modulation levels is increased. Thus, the multilevel modulation scheme is advantageous in that high-speed information can be sent within a limited bandwidth. However, the multilevel modulation scheme, if it experiences a fading environment, has abrupt performance degradation. Generally, the mobile communication system is configured so that several mobile stations (MSs) communicate with one another via one base station (BS). However, in the mobile communication system, the phase of a received signal can become distorted due to a fading phenomenon occurring on a radio channel during high-speed data transmission. The fading reduces the amplitude of a received signal by several dB to several tens of dB. If a phase of a received signal distorted due to the fading phenomenon is not compensated for during data demodulation, the phase distortion becomes an information error cause of transmission data transmitted by a transmission side, causing a reduction in quality of a mobile communication service. Therefore, in order to transmit high-speed data without a decrease in the service quality, the mobile communication system must overcome fading, and various diversity techniques have been proposed to cope with performance degradation due to the fading.
Generally, a CDMA mobile communication system adopts a rake receiver that performs diversity reception by using delay spread of a channel. While the rake receiver applies reception diversity for receiving a multipath signal, a rake receiver applying the diversity technique using the delay spread is disadvantageous in that it does not operate when the delay spread is less than a preset value. In addition, a time diversity technique using interleaving and coding is used in a Doppler spread channel. However, the time diversity technique is disadvantageous in that it can hardly be used in a low-speed Doppler spread channel.
Therefore, in order to compensate for the fading, a space diversity technique is used in a channel with low delay spread, such as an indoor channel, and a channel with low-speed Doppler spread, such as a pedestrian channel. The space diversity technique uses two or more transmission/reception antennas. In this technique, when a signal transmitted via one transmission antenna decreases in its signal power due to fading, a signal transmitted via the other transmission antenna is received. The space diversity can be classified into a reception antenna diversity technique using a reception antenna and a transmission diversity technique using a transmission antenna. However, since the reception antenna diversity technique is applied to a mobile station, it is difficult to install a plurality of antennas in the mobile station in view of a size of the mobile station and its installation cost. Therefore, it is recommended that the transmission diversity technique should be used in which a plurality of transmission antennas are installed in a base station.
Particularly, in a 4th generation mobile communication system, a data rate of about 10 Mbps to 150 Mbps is expected, and an error rate requires a bit error rate (hereinafter referred to as “BER”) of 10−3 for voice, BER of 10−6 for data, and BER of 10−9 for image. The STTC is based on a combination of a multi-antenna technique and a channel coding technique, and is a technique bringing a drastic improvement of a data rate and reliability in a radio MIMO (Multi Input Multi Output) channel.
In addition, a space-time code (hereinafter referred to as “STC”), based on a combination of a multi-antenna scheme and a channel coding scheme, is a code capable of improving frequency efficiency and reliability in a radio environment. The STC allows a receiver to obtain space-time diversity gain by extending a transmission signal to a 2-dimensional area of time and space. In addition, a coding scheme using the STC, unlike the existing channel coding scheme, can obtain coding gain without additional extension of a bandwidth by accommodating redundancy generated due to coding in a space-time dimension, thereby contributing to a remarkable improvement in channel capacity.
The STTC, a kind of the STC, obtains a receiver's space-time diversity gain by extending a space-time dimension of a transmitter's transmission signal. In addition, the STTC can obtain coding gain without a supplemental bandwidth, contributing to a large improvement in channel capacity. Therefore, in the transmission diversity technique, the STTC is used. When the STTC is used, coding gain having an effect of amplifying transmission power is obtained together with diversity gain which is equivalent to a reduction in channel gain occurring due to a fading channel when the multiple transmission antennas are used. A method for transmitting a signal using the STTC is disclosed in Vahid Tarokh, N. Seshadri, and A. Calderbank, “Space Time Codes For High Data Rate Wireless Communication: Performance Criterion And Code Construction,” IEEE Trans. on Info. Theory, 1998, 3, (2), pp. 744-765, the contents of which are incorporated herein by reference.
In the above reference, Vahid Tarokh has proposed a pairwise error rate of STTC in a fast Rayleigh fading environment, and STTC design criteria based on the pairwise error rate. Vahid Tarokh set a design criterion for coding gain among the STTC design criteria by a minimum product distance among all codewords corresponding to a minimum effective length. Thereafter, in a paper entitled “Space Time TCM With Improved Performance On Fast Fading Channel” submitted by Firmanto. W., Vucetic. B. S., and Yuan. J. (see Firmanto. W., Vucetic. B. S., Yuan. J., “Space Time TCM With Improved Performance On Fast Fading Channel,” IEEE Commun. Lett., 2001, 4, (4), pp. 154-156), Firmanto has proposed STTC that optimally satisfies a minimum product distance design criterion of Vahid Tarokh. Afterward, in a paper entitled “Space Time Codes For Fading Channels” submitted by Yongacoclu. A., and Siala. M. (see Yongacoclu. A., Siala. M., “Space Time Codes For Fading Channels”, Proc. VTC, Rhodes, Greece, 2001, pp 1132-1136), Yongacoclu has proposed a method of concatenating a space time block code (hereinafter referred to as “STBC”) and an existing trellis code modulation (hereafter referred to as “TCM”) scheme in order to maximize coding gain and diversity gain, and has asserted that the method of concatenating the STBC and the TCM scheme can obtain higher diversity gain and coding gain as compared with when only the STTC is used.
It will be assumed herein that in a mobile communication system using STC, a transmitter transmits a signal through two transmission antennas and a receiver receives a signal through one reception antenna. In such a mobile communication system, a signal received at a particular time t is represented by
                              r          t                =                                                            E                S                                      ⁢                                          ∑                                  i                  =                  1                                2                            ⁢                                                h                  t                  i                                ⁢                                  c                  t                  i                                                              +                      n            t                                              Equation        ⁢                                  ⁢                  (          1          )                    
In Equation (1), rt denotes a signal received at a particular time t, hit denotes a complex fading coefficient from an ith transmission antenna with mean zero and variance 0.5 per dimension, ES denotes energy per symbol, cit denotes a space-time coded symbol transmitted via an ith transmission antenna, and nt denotes a complex Gaussian noise with mean zero and variance NO/2 per dimension.
A description will now be made of a comparison between a case where the method of concatenating STBC and a TCM scheme, proposed by Yongacoclu, is used and a case where only STTC is used in the mobile communication system sated above.
First, with reference to FIG. 1, a description will be made of the case where the method of concatenating STBC and a TCM scheme, proposed by Yongacoclu, is used in the mobile communication system.
FIG. 1 is a block diagram illustrating a general transceiver structure of a mobile communication system using the method of concatenating STBC and a TCM scheme. Referring to FIG. 1, when information data bits are received, the information data bits are provided to a TCM encoder 111. Although a TCM scheme is used as an encoding scheme in FIG. 1, a multiple trellis coded modulation (hereinafter referred to as “MTCM”) scheme can also be used as an encoding scheme. The TCM encoder 111 encodes the received information data bits in the TCM scheme, and then provides the encoding result to a symbol interleaver 113. Here, it is assumed that a signal output from the TCM encoder 111 is an M-ary symbol, and the M-ary symbol, i.e., a codeword, is x=(x1, x2, . . . , xt, . . . , x1). The symbol interleaver 113 interleaves a codeword x=(x1, x2, . . . , xt, . . . , x1) output from the TCM encoder 111 in a predetermined interleaving scheme, and then provides the interleaving result to an STBC encoder 115. A block size of the symbol interleaver 113 is N for a symbol duration, and is defined by the predetermined interleaving scheme, i.e., a mapping function f(t).
The STBC encoder 115 encodes a signal output from the symbol interleaver 113 into STBC, and then transmits the encoding result to a receiver through the two transmission antennas, i.e., a first transmission antenna Tx.ANT1 and a second transmission antenna Tx.ANT2. For example, when an output of the symbol interleaver 113 was s0s1, the STBC encoder 115 encodes the s0s1, into STBC, and outputs symbols (s0s1) and (−s1*,s0*) as shown in Table 1 below.
TABLE 1Tx.ANT1Tx.ANT2ts0s1t + T−s1*s0*
In Table 1, t represents a particular time, and t+T represents a time when a time T has passed since the particular time t.
Signals transmitted through the first transmission antenna Tx.ANT1 and the second transmission antenna Tx.ANT2 experience a radio channel environment. Therefore, as described in conjunction with Equation (1), both a channel that the signal transmitted via the first transmission antenna Tx.ANT1 experiences and a channel that the signal transmitted via the second transmission antenna Tx.ANT2 experiences have a complex fading coefficient hit. It will be assumed that a complex fading coefficient hit of the channel that the signal transmitted via the first transmission antenna Tx.ANT1 experiences and a complex fading coefficient hit of the channel that the signal transmitted via the second transmission antenna Tx.ANT2 experiences have continuity between two consecutive symbols for STBC decoding, i.e., hi2n−1=hi2n.
Meanwhile, a receiver receives a signal transmitted by the transmitter through one reception antenna Rx.ANT, and the received signal is provided to an STBC de coder 117. The STBC decoder 117 decodes the received signal with STBC, and then provides the decoding result to a symbol deinterleaver 119. The symbol deinterleaver 119 deinterleaves a signal output from the STBC decoder 117 according to the interleaving scheme applied in the transmitter, and then provides the deinterleaving result to a TCM decoder 121. Since the interleaving scheme applied in the transmitter is a mapping function f(t), the symbol deinterleaver 119 deinterleaves the signal output from the STBC decoder 117 according to an inverse function, f1(t), of the mapping function f(t). The TCM decoder 121 decodes a signal output from the symbol deinterleaver 119 in a TCM scheme, and outputs information data bits. The TCM scheme is used as a decoding scheme, since it is assumed in FIG. 1 that the TCM scheme is used as an encoding scheme. However, an MTCM scheme can be used as the decoding scheme when the MTCM scheme is used as the encoding scheme.
If it is assumed that when the codeword x=(x1, x2, . . . , xt, . . . , x1) is transmitted, a length of the codeword x is 1 and xt is a trellis coded symbol, then pairwise error probability that a maximum likelihood decoder can select a defective codeword x′=(x1′, x2′, . . . , xt′, . . . , x1′) is represented by
                                          p            1                    ⁡                      (                                          x                ->                                                      x                    ′                                    |                                      h                    1                                                              ,                              h                2                                      )                          =                  exp          (                                    -                                                E                  S                                                  4                  ⁢                                      N                    O                                                                        ⁢                                          ∑                                  t                  ∈                  η                                            ⁢                                                (                                                                                                                                      h                                                                                    f                                                              -                                1                                                                                      ⁡                                                          (                              t                              )                                                                                1                                                                                            2                                        +                                                                                                                    h                                                                                    f                                                              -                                1                                                                                      ⁡                                                          (                              t                              )                                                                                2                                                                                            2                                                        )                                ⁢                                                                                                                        x                        t                                            -                                              x                        t                        ′                                                                                                  2                                                              )                                    Equation        ⁢                                  ⁢                  (          2          )                    
In Equation (2), hi is assumed as hi=(hi1hi2 . . . hit . . . hiN), and η indicates a set of all ‘t's with xt≠xt’. Generally, it is assumed that a codeword length 1 is set to a value much less than a block size N of an interleaver, i.e., the symbol interleaver 113, (1<<N), and for all ‘t's with tεη, |hƒ−1(t)1| and |hƒ−1(t)2| are independent samples of Rayleigh distribution random variables on fast fading channels. If an average of Equation (2) is calculated through a probability density function (PDF) of h1 and h2, the pairwise error probability is represented by
                                                                                          p                  1                                ⁡                                  (                                      x                    ->                                          x                      ′                                                        )                                            ≤                            ⁢                                                ∏                                      t                    ∈                    η                                                  ⁢                                                      (                                          1                      +                                                                                                    E                            S                                                                                4                            ⁢                                                          N                              O                                                                                                      ⁢                                                                                                                                                                        x                                t                                                            -                                                              x                                t                                ′                                                                                                                                          2                                                                                      )                                                        -                    2                                                                                                                          ≤                            ⁢                                                ∏                                      t                    ∈                    η                                                  ⁢                                                      (                                                                                            E                          S                                                                          4                          ⁢                                                      N                            O                                                                                              ⁢                                                                                                                                                            x                              t                                                        -                                                          x                              t                              ′                                                                                                                                2                                                              )                                                        -                    2                                                                                                          Equation        ⁢                                  ⁢                  (          3          )                    
Secondly, a description will be made of the case where STTC is used in the mobile communication system.
Unlike when the STBC is concatenated with the TCM scheme, when the STTC is used, STTC encoding is performed so the cit is an STTC coded symbol. If a codeword c=(c1, c2, . . . , ct, . . . , c1) is transmitted and it is assumed that ct=c1tc2t, then pairwise error probability that a maximum likelihood decoder can select a defective codeword c′=(c1′, c2′, . . . , ct′, . . . , c1′) is represented by
                                          p            1                    ⁡                      (                          c              ->                              c                ′                                      )                          ≤                              ∏                          t              ∈              η                                ⁢                                    (                                                                    E                    S                                                        4                    ⁢                                          N                      O                                                                      ⁢                                                                                                                        c                        t                                            -                                              c                        t                        ′                                                                                                  2                                            )                                      -              1                                                          Equation        ⁢                                  ⁢                  (          4          )                    
In Equation (4), η represents all ‘t's with ∥ct−ct′∥≠0.
As described above, the result asserted by Yongacoclu is equal to the result obtained through simulation of a Monte-Carlo technique, and there is not enough theoretical basis regarding that the method of concatenating STBC and a TCM scheme affects which parameter among system parameters, thus improving performance. According to the minimum product distance design criterion actually proposed by Vahid Tarokh, the method proposed by Yongacoclu cannot explain a performance difference between STTCs. Accordingly, there is a demand for an STTC generation method for achieving both diversity gain and coding gain even in a fast fading environment.